Earlier studies designed to test the toxicological sensitivity of various compounds have shown marked differences in the sensitivity between the various strains used. Our present studies use known chemical mutagens and are designed to confirm the above finding as well as determine variability of mutagenic sensitivity between routes of administration and between and within strains of mice. Mice of two inbred strains and/one hybrid are each given by either i.p. injection or gavage three different dose levels of ethyl methanesulfonate (EMS). Similarly male mice of one inbred strain are given an injection of triethylenemelamine. All mice are observed for the toxic effects of the compounds and subsequently mated according to the standard dominant lethal assay.